1. Field of the Invention
The invention is related to the ammonothermal growth of group-III nitride crystals on seeds with at least two surfaces making an acute, right or obtuse angle with each other.
2. Description of the Related Art
Ammonothermal growth of group-III nitrides, for example, gallium nitride (GaN), involves placing, within a reactor vessel, group-III containing source material, group-III nitride seed crystals, and a nitrogen-containing fluid or gas, such as ammonia, then sealing the vessel and heating the vessel to conditions, such that the reactor is at elevated temperatures (between 23° C. and 1000° C.) and high pressures (between 1 atm and, for example, 30,000 atm). Under these temperatures and pressures, the nitrogen-containing fluid becomes a supercritical fluid and normally exhibits enhanced solubility of group-III nitride material into solution.
The solubility of group-III nitride into the nitrogen-containing fluid is dependent on the temperature, pressure and density of the fluid, among other things. By creating two different zones within the vessel, it is possible to establish a solubility gradient where the solubility in a first zone will be higher than the solubility in a second zone. The source material is then preferentially placed in the higher solubility first zone and the seed crystals in the lower solubility second zone. By establishing fluid motion between these two zones, for example, by making use of natural convection, it is possible to transport group-III nitride material from the higher solubility first zone to the lower solubility second zone where it then deposits itself onto the seed crystals.
Currently, when growing group-III nitride crystals using the ammonothermal method, it may be possible that the growth along one crystallographic direction is slower than along another crystallographic direction. What may be seen when growing, for example, GaN, is that the growth rate along the polar c-direction {0001} is approximately four to ten times faster than along a perpendicular, stable, non-polar direction, such as the m-direction {10-10}. Additionally, the absolute growth rate along the stable, non-polar direction may be relatively small, on the order of 10-50 μm/day. In order to fabricate substrates from bulk group-III nitride crystals, it is desirable to obtain the highest possible growth rates, while still maintaining crystal quality.
If it is desired to produce substrates which have a large non-polar and/or semi-polar surface, it is desirable to have rapid growth rates along the c-direction, but also along a perpendicular non-polar direction. It has been observed that growth in the non-polar a-direction {11-20}, which is perpendicular to both the m-direction and c-direction, can be up to 10 times faster than the m-direction. Additionally, the a-plane typically grows out of existence and multiple m-plane facets form, replacing the original a-plane surface of the seed.
In addition to improving on the overall growth rate of the crystal, it is important to control the uptake of chemical species. Unless a perfectly pure environment exists during growth, which is never the case, chemicals present in the growth chamber will be incorporated into the growing GaN crystal. These chemicals may modify the properties of the crystals in a good way (e.g. dopants to improve on the free carrier concentrations, etc.) or a bad way (e.g. impurities reducing overall carrier lifetime or optical properties by introducing electronic states within the band gap of the crystal, etc.).
Currently, once a crystal of a certain size is obtained, the surfaces that grow in steady state are typically m-plane {10-10} and c-plane {0001} (although occasionally, the {10-11} or {10-1-1} planes are also seen). The uptake of chemicals along these planes is dominated by the interface between the crystal and the environment and, since only a limited number of stable planes emerge (e.g., m- and c-planes), the uptake of chemicals in the resulting crystal will be dominated by the uptake of chemicals on the m- and c-planes.
What is needed, then, is an improved method of fabricating III-nitride-based crystals that solves or reduces these problems. The present invention satisfies this need.